FIG. 1 is a cross-sectional view of a typical prior art pressure sensor 10 useful for measuring full scale pressures on the order of 1000 Torr or less; sometimes referred to as a vacuum manometer or capacitance diaphragm gauge. The body 12 of the sensor 10 may be formed of a high nickel alloy, stainless steel, or other engineering material in order to provide protection against induced corrosion. A thin metallic diaphragm 14 is disposed between the body 12 and the lower body 13 and separates a hollow space within the body 12 into a measured pressure cavity 16 and a reference pressure cavity 18. The measured pressure cavity 16 is in fluid communication with a measured pressure that is being communicated via an inlet tube 20. The inlet tube 20 is held in place by a weld joint 21 or similar method such as brazing. The pressure in the reference pressure cavity 18 is reduced to a controlled low reference pressure by a vacuum pump or other vacuum source (not shown) via evacuation tube 22. The evacuation tube 22 is held in place by a braze joint 23 or similar joining method such as solder. The low reference pressure is maintained in the reference pressure cavity 18 by a getter pump 24, which is a reactive material which absorbs gas molecules from the reference pressure cavity 18 by chemical reaction or adsorption. The getter pump 24 is separated from the reference pressure cavity 18 by a filter screen 25. Disposed above the diaphragm 14 and separated from it by a gap G is a sensing electrode 26 made of an electrically conductive material. The conductive diaphragm 14 and the electrode 26 function together as a capacitor to provide a capacitance signal to sensing circuitry 28 of the pressure sensor 10. The electrode 26 includes an electrode post 27 which is electrically insulated from the body 12 of the sensor 10 by a compression glass to metal seal 30. One skilled in the art will appreciate that as the measured pressure changes, there is a corresponding pressure change in the measured pressure cavity 16, which in turn causes the diaphragm 14 to deflect, changing the size of the gap G and the value of the capacitance measured between the diaphragm 14 and the electrode 26. The sensing circuitry 28 is calibrated to convert a measured capacitance to an output signal indicative of the pressure applied in the measured pressure cavity 16.
Any variable that affects the size of gap G will be detected and indicated as a change in the measured pressure. Variables other than applied pressure that affect the size of gap G are sources of error in the electrical output signal of the vacuum manometer. One such variable is temperature. Different materials used to construct the various components of the sensor 10 have different coefficients of thermal expansion (CTE), thereby causing relative dimensions within the sensor 10 to vary with temperature changes. The use of a hermetic compression seal between the body 12 and the electrode post 27 creates one source of this type of measurement error. As stated above, the material of the body 12 is selected for its corrosion resistance and generally has a relatively high CTE. The hermetic joint between the body 12 and the electrode post 27 is achieved by heating the assembled body 12/glass seal 30/electrode post 27 combination to a temperature high enough to soften the glass seal 30, thereby allowing it to flow into complete conforming contact between the body 12 and electrode post 27. As the temperature is cooled back to ambient, the glass seal 30 hardens and becomes compressed between the body 12 and the electrode post 27 due to the relatively high CTE of the body 12 and relatively lower or matched CTE of the electrode post 27. The material of the glass seal 30 is selected to be electrically insulating and to have a CTE that is lower than that of the material of the body 12, but higher than or matched with the electrode post 27. The material of the electrode post 27 is selected to be electrically conductive and to have a CTE that is lower than or matched with that of both the body 12 and the glass seal 30, and may be for example a nickel-iron alloy. Thus, as the assembled body 12/glass seal 30/electrode post 27 combination cools, the body 12 compresses onto the glass 30, which in turn compresses onto the electrode post 27, thereby creating a hermetic seal. As the temperature of the sensor 10 changes during operation, the relatively high CTE body 12 and the relatively low CTE electrode post 27 will experience differential thermal growth, thereby changing gap G and introducing an error into the output signal. Prior art devices are known to compensate for such error by measuring the temperature and applying a correction algorithm within the sensing circuitry 28.
In another example of prior art, a scheme with a dual electrode is used such as described in U.S. Pat. No. 4,823,603. This approach uses two electrodes screened onto a flat ceramic substrate which is parallel and offset at a distance from a thin metal foil diaphragm. The diaphragm is supported along its periphery and it displaces in the shape of a dome toward the electrodes as the measured pressure increases. The dual electrode design allows for the elimination of some measurement errors by the subtraction of the reference capacitance from the sensing capacitance. This approach adds incremental cost and manufacturing difficulty over the single electrode design of FIG. 1.